U.S. patent application number 10/040513 was filed with the patent office on 2003-03-27 for ovsf code system and methods.
This patent application is currently assigned to InterDigital Technology Corporation. Invention is credited to Kim, Younglok, Pan, Jung-Lin.
Application Number | 20030058788 10/040513 |
Document ID | / |
Family ID | 26717132 |
Filed Date | 2003-03-27 |
United States Patent
Application |
20030058788 |
Kind Code |
A1 |
Kim, Younglok ; et
al. |
March 27, 2003 |
OVSF CODE SYSTEM AND METHODS
Abstract
A code indexing method for orthogonal variable spreading factor
(OVSF) codes introduces a single number mapped to the each code.
The new code number itself not only provides the code signature,
but it is also used for the OVSF code generation. In addition, it
provides easy and fast generation of the available code list
without the help of look-up table. This capability improves the
dynamic code assignment.
Inventors: |
Kim, Younglok; (Fort Lee,
NJ) ; Pan, Jung-Lin; (Selden, NY) |
Correspondence
Address: |
VOLPE AND KOENIG, PC
DEPT ICC
SUITE 400, ONE PENN CENTER
1617 JOHN F. KENNEDY BOULEVARD
PHILADELPHIA
PA
19103
US
|
Assignee: |
InterDigital Technology
Corporation
Wilmington
DE
|
Family ID: |
26717132 |
Appl. No.: |
10/040513 |
Filed: |
December 28, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60323120 |
Sep 18, 2001 |
|
|
|
Current U.S.
Class: |
370/208 ;
370/209 |
Current CPC
Class: |
H04J 13/0044 20130101;
H04J 13/0048 20130101; H04J 13/12 20130101; H04J 13/20
20130101 |
Class at
Publication: |
370/208 ;
370/209 |
International
Class: |
H04J 011/00 |
Claims
What is claimed is:
1. In a communications system where OVSF codes are selected from a
set of Walsh codes represented as a binary tree having multiple
layers such that a zeroth layer has one Walsh code and each
successive layer has twice the number of Walsh codes as the layer
it succeeds, the method comprising: selecting an OVSF code from the
set of Walsh codes using an index p where p represents the
(p+1)-2.sup.ith Walsh code of the ith layer of Walsh codes where i
is an integer such that 2.sup.i.ltoreq.p<2.sup.i+1.
2. The method according to claim 1 wherein the OSVF code is
selected on the basis of a spreading factor SF which is a power of
2 and a Walsh code is selected having an associated index p where
SF.ltoreq.p<2SF .
3. The method of claim 1 further comprising determining the
relative orthogonality of the selected Walsh code of layer i
represented by index value p with another Walsh code of layer j
represented by an index value q by comparing the binary forms of p
and q.
4. The method of claim 3 wherein the binary form of p is a sequence
of i significant binary digits and the binary form of q is a
sequence of j significant binary digits and the represented Walsh
codes are determined to be not orthogonal if either the binary form
of p is the same as the i most significant binary digits of the
binary form of q or the binary form of q is the same as the j most
significant binary digits of the binary form of p.
5. The method of claim 1 further comprising generating a selected
Walsh code represented by index value p based upon a sequence of
significant binary digits representing the binary form of p.
6. The method of claim 5 wherein the selected Walsh code is
generated as the Kronecker Product of i Walsh codes represented by
index values 2 and 3 correspondingly to the sequence of i
significant binary digits of the binary form of p where each binary
digit 0 corresponds to the Walsh code of index value 2 and each
binary digit 1 corresponds to the Walsh code of index value 3.
7. The method of claim 5 wherein the selected Walsh code is
generated by the Kronecker product of 2 Walsh codes represented by
index values q and r of respective layers of j and k where
j+k=i.
8. The method of claim 7 wherein the binary form of p is the same
as the binary form of q concatenated with the binary forms of
(r-2.sup.k).
9. In a communications system where OVSF codes are used and
selected based upon a spreading factor SF where SF is a positive
power of 2, the method comprising: selecting an OVSF code using an
index p from a set of codes where: for each integer p>3 the
corresponding code is defined by C(p)=C(m+2){circle over (x)}C(k),
with p=2.multidot.k+m, where k and m are integers with m=0 or 1,
and the codes corresponding to p=1, 2, or 3 are C(1)=[1],
C(2)=[1,1], and C(3)=[1,-1]whereby each p represents the
(p+1)-2.sup.ith code of an ith layer of codes for SF=2.sup.i where
i is the unique integer such that
2.sup.i.ltoreq.p<2.sup.i+1.
10. The method according to claim 9 further comprising determining
the relative orthogonality of a selected code of layer i
represented by index value p with another code of layer j
represented by an index value q by comparing the binary forms of p
and q.
11. The method of claim 10 wherein the binary form of p is a
sequence of i significant binary digits and the binary form of q is
a sequence of j significant binary digits and the represented codes
are determined to be not orthogonal if either the binary form of p
is the same as the i most significant binary digits of the binary
form of q or the binary form of q is the same as the j most
significant binary digits of the binary form of p.
12. The method of claim 9 further comprising generating a selected
code represented by index value p based upon a sequence of
significant binary digits representing the binary form of p.
13. The method of claim 12 wherein the selected code is generated
as the Kronecker Product of i codes represented by index values 2
and 3 correspondingly to the sequence of i significant binary
digits of the binary form of p where each binary digit 0
corresponds to the code of index value 2 and each binary digit 1
corresponds to the code of index value 3.
14. The method of claim 12 wherein the selected code is generated
by the Kronecker product of two codes represented by index values q
and r of respective layers of j and k where j+k=i.
15. The method of claim 14 wherein the binary form of p is the same
as the binary form of q concatenated with the binary forms of
(r-2.sup.k).
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims priority from U.S. Provisional
Patent Application Serial No. 60/323,120, filed Sep. 18, 2001.
FIELD OF INVENTION
[0002] The present invention relates CDMA communication systems
and, in particular, to Orthogonal Variable Spreading Factor (OVSF)
codes and methods for allocating, generating and determining
orthogonality of OVSF codes of different data rates.
BACKGROUND
[0003] Orthogonal variable spreading factor (OVSF) codes provide an
orthogonal code set of variable spreading factors. In the prior
art, methods exist for allocating a set of OVSF codes of different
data rates employing Walsh codes of variable length. The code
assignment is made on the basis of channel data rates in a manner
that results in improved utilization of the available frequency
spectrum.
[0004] An alternative method to obtain OVSF codes based on the code
tree structure is based on the modified Hadamard transformation,
which requires two indices to indicate a specific code, (i.e.,
spreading factor and code number). In order to handle the code
allocation process, an ASSIGNED list and a BUSY conventionally
generated.
[0005] These prior art methods have drawbacks in that they require
a large amount of memory to store a large number of codes, or
require fast processing speeds to generate the codes or effectively
allocate the available codes.
SUMMARY
[0006] A code indexing system and method for orthogonal variable
spreading factor (OVSF) codes introduces a single number mapped to
the each code. The new code number itself not only provides the
code signature, but it is also used for the OVSF code generation.
In addition, it provides easy and fast generation of the available
code list without the help of look-up table. This capability
improves the dynamic code assignment.
[0007] OVSF codes are selected from a set of Walsh codes by using
an index p where p represents the (p+1)-2.sup.ith Walsh code of the
ith layer of Walsh codes where i is an integer such that
2.sup.i.ltoreq.p<2.sup.i+1- . Preferably, the OSVF code is
selected on the basis of a spreading factor SF which is a power of
2 and a Walsh code is selected having an associated index p where
SF.ltoreq.p<2SF.
[0008] The relative orthogonality of a selected Walsh code of layer
i represented by index value p with another Walsh code of layer j
represented by an index value q is determined by comparing the
binary forms of p and q. The binary form of p is a sequence of i
significant binary digits and the binary form of q is a sequence of
j significant binary digits. The represented Walsh codes are
determined to be not orthogonal if either the binary form of p is
the same as the i most significant binary digits of the binary form
of q or the binary form of q is the same as the j most significant
binary digits of the binary form of p.
[0009] A selected Walsh code represented by index value p is easily
generated based upon the sequence of significant binary digits
representing the binary form of p. The selected Walsh code is
generated as the Kronecker Product of i Walsh codes represented by
index values 2 and 3 correspondingly to the sequence of i
significant binary digits of the binary form of p where each binary
digit 0 corresponds to the Walsh code of index value 2 and each
binary digit 1 corresponds to the Walsh code of index value 3.
[0010] Alternatively, the selected Walsh code is generated by the
Kronecker product of two Walsh codes represented by index values q
and r of respective layers of j and k where j+k=i. In such case,
the binary form of p is the same as the binary form of q
concatenated with the binary forms of (r-2.sup.k).
[0011] In general, OVSF codes are used and selected based upon a
spreading factor SF where SF is a positive power of 2, using an
index p from a set of codes where for each integer p>3 the
corresponding code is defined by C(p)=C(m+2){circle over (x)}C(k),
with p=2.multidot.k+m, where k and m are integers with m=0 or 1.
The codes corresponding to p=1, 2 or 3 are C(1)=[1], C(2)=[1, 1],
and C(3)=[1, -1]. Accordingly, p represents the (p+1)-2.sup.ith
code of an ith layer of codes for SF=2.sup.i where i is the unique
integer such that 2.sup.i.ltoreq.p<2.sup.i+1.
[0012] Other objects and advantages of the invention will be
apparent to those skilled in the art from the following
description.
BRIEF DESCRIPTION OF THE DRAWING(S)
[0013] FIG. 1 is a prior art OVSF code tree of Walsh codes.
[0014] FIG. 2 is a table representing an indexing system according
to the teaching of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
[0015] A conventional OVSF code tree structure is shown in FIG. 1
which codes are referred to as Walsh codes herein. C.sub.SF(n)
denotes the OVSF code word with the spreading factor SF=2.sup.k,
where n is the code number and k is the layer number. The indices n
and k are known as the Hadamard indices. The Walsh codes are
conventionally generated recursively from the code tree as shown in
FIG. 1.
[0016] The mother codes are the lower layer codes on the path from
the specific code to the root code C.sub.1(0), and the descendent
codes are those produced from the specific code. For example, the
mother codes of C.sub.8(2) is C.sub.4(1), C.sub.2(0) and
C.sub.1(0), and the descendent codes of C.sub.4(1) are C.sub.8(2),
C.sub.8(3) and their descendent codes.
[0017] Two codes are orthogonal if, and only if, any one is not the
mother code or the descendent code of another. When a specific code
is assigned, its mother codes and descendent codes cannot be
assigned in the same channel since they are not orthogonal to each
other. In other words, two OVSF codes with different spreading
factors are not orthogonal when they are on the same branch of the
code tree.
[0018] When a new call is requested with a specific data rate, the
system needs to assign a code from an available set of codes with
the corresponding spreading factor. Conventionally, in order to
maintain the orthogonality between assigned codes, the set of
available code list is updated whenever the new code is assigned.
This code set is updated by removing the assigned code itself and
all of its descendent and mother codes.
[0019] The inventors have recognized that the prior art codes can
each be assigned via a single indice system instead of the dual
indice system per the known Hadamard method. In the single indice
system of the present invention, a sequential numerical code label
p is assigned where p equals the sum of the code layer plus the
code number of the conventional tree structure designation using
Hadamard indices. As such, the code labels are sequential integers
starting with the one code of layer 0 where SF=1 labeled as 1, the
two codes of layer 1 where SF=2 labeled as 2 and 3, followed by the
2.sup.i codes of layer i where SF=2.sup.i labeled as the next
2.sup.i integers for each successive layer i from 2 onward as
represented in FIG. 2 for layers 0 through 3. Although only codes
for spreading factors up to 8 are shown in FIG. 2, the system is
applicable for spreading codes of any power of 2.
[0020] In general for each positive integer label p, there is a
unique integer i, where 2.sup.i.ltoreq.p<2.sup.(i+1), and p
represents the (p+1)-2.sup.i th Walsh code of the ith layer of
Walsh codes. For example, when p=87, i=6 since 64.ltoreq.p<128,
so that 87 represents the 24th Walsh code of the 6th layer of the
Walsh codes. For p=1, i=0 since 2.sup.0.ltoreq.1<2, so 1
represents the first code of the zeroth layer. Generally, for a
prior art code designated C.sub.N(x) , that code is the (x+1) th
code of layer N, since the prior art code designations start for
each layer with x=0.
[0021] In stead of using the prior art designations of FIG. 1, the
prior art tree-structured codes can be generated for each positive
integer p by the recursive Kronecker procedure where for each
integer p>3 the corresponding code is defined by:
C(p)=C(m+2){circle over (x)}C(k), Equation (1)
[0022] with
p=2.multidot.k+m, Equation (2)
[0023] where k and m are integers with m=0 or 1, and the codes
corresponding to p=1, 2 or 3 are:
C(1)=[1], C(2)=[1, 1], and C(3)=[1, -1]. Equation (3)
[0024] As noted above, for any specified p there is a unique
integer i such that 2.sup.i.ltoreq.p<2.sup.i+1, so that each p
represents a code of only one SF, namely SF=2.sup.i. Also, the code
represented by p is the (p+1)-2.sup.ith code of an ith layer of
codes starting with p=1 representing the first code of a zeroth
layer.
[0025] Codes generated in this manner meet the following three
properties:
[0026] Property 1: The OVSF code for code label p where
SF.ltoreq.p<2SF and SF=2.sup.L can be factored into a Kronecker
product with L terms of C(2) or C(3) as follows:
C(p)=C(a.sub.L-1+2){circle over (x)}. . . {circle over
(x)}C(a.sub.1+2){circle over (x)}C(a.sub.0+2) Equation (4)
[0027] where a.sub.0=1 and each a.sub.i, for i=1 to L-1, is 0 or 1
and 1 p = a 0 2 L - 1 + a 1 2 L - 2 + + a L - 1 = i = 0 L - 1 ( a i
2 ( L - i - 1 ) ) Equation ( 5 )
[0028] Thus, a.sub.0a.sub.1 . . . a.sub.L-1 is the binary
representation of p where a.sub.0=1 and each a.sub.i, for i>1,
is the binary digit 1 or 0.
[0029] Property 2: The mother codes of C(p) are all of the
form:
[C(a.sub.L-m+2){circle over (x)} . . . {circle over
(x)}C(a.sub.1+2){circle over (x)}C(a.sub.0+2)]
[0030] with m=2,3, . . . L.
[0031] Property 3: The descendent codes of C(p) are all C(q){circle
over (x)}C(p) with any positive integer q.
[0032] For notational purposes, code designations of the present
invention C(p) where p is in decimal form can also be represented
as c(p.sub.binary) ,i.e. c(a.sub.0 . . . a.sub.N-1) where a.sub.0 .
. . a.sub.N-1 is the binary representation of p. For example,
C(6)=c(110), since 6 in decimal notation equals 110 in binary
notation.
[0033] The code indexing system in accordance with the present
invention is illustrated in FIG. 2. The OVSF codewords with their
spreading factor up to 8 are shown with both the conventional index
using the Hadamard indices and the new code index
representations.
[0034] The OVSF code layer numbers are shown in the first column.
The conventional OVSF code indices are shown in the second column,
i.e., SF and code number. The third and fourth columns are the
binary and decimal forms of the code labels of the present
invention. The code label index maps a code label to each codeword
shown in the last column. The codewords of FIG. 2 correspond
directly to the Walsh code sets of 1s and -1s in FIG. 1 with 0s in
the codeword being substituted for each -1 of the corresponding
Walsh code.
[0035] In view of properties 2 and 3, the mother codes and
descendent codes of c(a0,a1,a2,a3) are {c(a0), c(a0,a1),
c(a0,a1,a2)} and all the codes having binary indices starting with
(a0,a1,a2,a3), i.e., c(a0,a1,a2,a3,X,X,X, . . . ) are easily
identifiable.
[0036] The code label indexing method in accordance with the
present invention has several distinct advantages over prior art
methods:
[0037] 1) Reduced Number of Bits for Identifying Codes and
Increased Capacity
[0038] The new indexing method needs only L+1 bits to support the
maximum spreading factor 2.sup.L, while the conventional indexing
needs L+[log.sub.2(L)-1] bits for the same case. For example, there
is a 3 bit saving for the maximum spreading factor 512. For maximum
spreading factor 512, the conventional method needs 4 bits to store
ten spreading factors {1,2,4,8,16,32,64,128,256,512} or ten layer
numbers {0,1,2,3,4,5,6,7,8,9} correspondingly. In addition, the
conventional method needs 9 bits to distinguish between the 512
codes in the 10th layer. Accordingly, a total of 13 bits are
conventionally required to identify a particular code within a 10
layer system which supports spreading factors up to and including
512. In comparison, the new method needs only 10 bits to
distinguish all the codes of 1023 for spreading factors up to and
including 512. The reduction of 3 bits from 13 bits represents a
nearly 25% increase in capacity.
[0039] 2) Easy to Generate Available Orthogonal Codes During Code
Assignment
[0040] With the new indexing, the available code with the specific
spreading factor can be generated directly in a straightforward way
from the binary form of the indices of the assigned code without
requiring the use of look-up tables.
[0041] For instance, if the code represented by 89 (or 1011001) is
assigned, its mother codes and descendent codes cannot be assigned
for the use simultaneously to maintain relative orthogonality of
used codes. Those codes would normally be marked "BUSY" when code
89 is used. The BUSY codes are easily generated because mother
codes of code#89 are code#70 (101100), code#22 (10110), code#11
(1011), code#5 (101), code#2 (10), code#1 (1), an in a nine layer
system, are code#178 (10110010), code#179 (10110011), code#356
(101100100), code#357 (101100101), code#358 (101100110), code#359
(101100111) according to properties 2 and 3.
[0042] In general, each code index has a binary form which is
represented by a sequence of significant binary digits of a length
equal to the layer of the Walsh code it represents. To determine
the relative orthogonality of one Walsh code of layer i,
represented by index value p, with another Walsh code of layer j,
represented by an index value q, the binary forms of p and q are
compared. Since the binary form of p is a sequence of i significant
binary digits and the binary form of q is a sequence of j
significant binary digits, the represented Walsh codes are not
orthogonal only if either the binary form of p is the same as the i
most significant binary digits of the binary form of q or the
binary form of q is the same as the j most significant binary
digits of the binary form of p. For p=87, this condition is true
only for q=1, 2, 5, 11, 22, 70, 178, 178, 356, 357, 358 or 359 in a
nine layer system as referenced above.
[0043] 3) Easy to Spread with the Long Code
[0044] The spread sequence with a long code can be obtained by the
multiple spreading with shorter spreading factors. The short
spreading code numbers are directly extracted from the long code
number.
[0045] For example, the spreading code c(a.sub.0,a.sub.1, . . . ,
a.sub.M) is the Kronecker product of c(a.sub.0,a.sub.1, . . . ,
a.sub.N) and c(a.sub.0,a.sub.N+1,a.sub.N+2, . . . , a.sub.M) with
N.ltoreq.M. Hence, the long spreading can be obtained by two
consecutive spreadings first with c(a.sub.0,a.sub.N+1,a.sub.N+2, .
. . , a.sub.M) and then with c(a.sub.0,a.sub.1, . . .
,a.sub.N).
[0046] 4) Easy to Generate the Long Code
[0047] The long code c(a.sub.0,a.sub.1, . . . , a.sub.M) can be
obtained by spreading c(a.sub.0,a.sub.1, . . . , a.sub.N) with
c(a.sub.0,a.sub.N+1,a.sub.N+2, . . . , a.sub.M). There is no
additional hardware complexity in generating the long code from the
shorter code.
[0048] For example, with references to FIG. 2:
c(1110)=c(11){circle over (x)}c(110) Equation (6)
[0049] since,
[1,-1,-1,1,1,-1,-1,1]=[1,-1]{circle over (x)}[1,-1,1,-1]
Equation(7)
[0050] Also:
c(1110)=c(111){circle over (x)}c(10) Equation (8)
[0051] since,
[1,-1,-1,1,1,-1,-1,1]=[1,-1-1,1]{circle over (x)}[1,1,] Equation
(9)
[0052] In general, any Walsh code of layer i, represented by an
index value p, can be generated by the Kronecker product of two
Walsh codes of layers j and k represented by respective index
values of q and r where j+k=i. In such case the binary form of p is
equal to the binary form of q concatenated with the binary form of
(r-2.sup.k).
[0053] 5) Reduced Memory Size for Code Table
[0054] The whole spreading code set does not have to be tabulated
in the memory. The above multi-stage spreading scheme needs a much
smaller table supporting a lower spreading factor. In addition,
there is no need to store the look-up table for the mother codes
and descendent codes of all codes. They can be generated in a
straightforward manner. For example, a 256 length OVSF code of
layer 8 can be generated by two 16 length OVSF codes of layer 4.
Hence a code table supporting codes up through layer 4 for a 16 SF
is enough to support the easy generation of all codes through layer
8 for a 256 SF. Alternatively, all spreading codes can be generated
using the layer two codes c(10) and c(11) per equation 4 above
where c(10)=C(2)=[1,1] and c(11)=C(3)=[1,-1].
[0055] 6) Enable Easy and Fast Dynamic Channel Assignment (DCA)
[0056] The indexing of the present invention benefits the dynamic
code assignment for easy and fast generation of AVAILABLE and BUSY
code lists. In conventional indexing methods, look-up tables are
required to store and search through all the mother codes and
descendent codes of all codes. Conventionally, the look-up tables
take up large amounts of memory and the searching process is
time-consuming.
[0057] With the new indexing method, there is no need for look-up
tables. All the mother codes and descendent codes can be obtained
directly in a straightforward manner from the assigned codes. This
enables a easy and fast dynamic code assignment.
[0058] Moreover, only a list of the index values of used codes need
be maintained to determine whether an orthogonal code is available
and to select such an orthogonal code. Where a code of spreading
factor SF is needed and prior used codes indexes p.sub.1 . . .
p.sub.n have been stored to a used code list, each value p from SF
through 2SF-1 can be compared against the stored used code index
values to determine the availability of an orthogonal code.
[0059] For simplicity, p can be first set equal to SF and the
binary form of p can be compared to the binary form of each of the
stored used code index values to determine orthogonality as set
forth above. If a comparison yields a determination of
non-orthogonality, the comparison process can be stopped, p
incremented by 1 and the comparison process repeated with the
incremented p. The process continues until a p is found which
represents a code orthogonal to all the used codes or until p is
incremented to equal 2SF. In the first case, the code corresponding
to p is selected for use as an orthogonal code and p is stored to
the set of used codes. In the second case where p is incremented to
equal 2SF, no orthogonal code is available.
[0060] The new code index method is a method for assigning single
number that indicates the layer number and the code number, and
moreover it indicates the structure of the code and the information
about the orthogonality to other codes.
* * * * *